The present invention relates to radio intruder alarm systems and is especially, though not exclusively, applicable to a microwave radio intruder alarm system.
A radio intruder alarm system of known kind includes a radio-frequency source for irradiating with radio-frequency radiation a space within which an intruder is to be detected. The known system also includes a radio detector responsive to said radiation returned towards said source from objects within said space or from the boundaries thereof, to yield detector signals representative of the amount of said returned radiation. The detector signals are applied to signal processing circuits responsive to said detector signals to yield an alarm signal in response to predetermined fluctuations in the amount of returned radiation.
An alarm system of the known kind described above may provide a false alarm in response to returned radiation reflected fluctuatingly from the ionized gas within illuminated fluorescent lighting tubes. Such spurious signals may be processed by the signal processing circuits as being fluctuations resulting from the presence of an intruder within the space to be protected.